1. Field of the Invention
The present invention relates to magnification devices which provide binocular optical magnification of images for the visually impaired.
2. Prior Art
Persons who have a visual impairment require magnification in order to be able to read magazine size prints or see other small images. Magnification can be delivered by a simple plus lens which magnifies or amplifies the image. The image is projected through the eye onto the retina. The expansion or enlargement of the image by such simple magnification will circumvent central visual acuity losses or central visual field losses. The amount of magnification required can be determined and in the prior art microscopes which can be either of a simple form such as a singular plus lens or of a complex form utilizing a plus lens doublet system and additional lenses have been utilized to provide this magnification.
With such prior art systems, a problem optically has been known for many years in that the closer the focal length of the lens, the more difficult it is to provide binocularity to the visually impaired person. This has been dealt with in simple microscope prescriptions utilizing a singular high plus lens before each eye by incorporating base-in prisms which shift the image temporarily for each eye, providing more comfort and because of reduced convergence demand. Generally, binocularity can be tolerated up to approximately +12 diopters (3.times.) by means of an appropriate amount of base-in prism to establish fusion; however, when using powers above +12 diopters, the comfort zones decrease considerably because the working distance or focal length of the lens decreases to less than 12 cm.
This problem of the prior art has been partially alleviated by the development of the tele-microscope which is a Galilean Telescope with a reading cap to provide a greater focal length. However, such tele-microscopes are further limited by their power and the extent or scope of their visual field.